Field of the Invention
Embodiments of the present disclosure relate generally to droop control and, more particularly, to droop control for a plurality of power converters.
Description of the Related Art
Droop control is an industry standard technique for autonomously sharing load among parallel AC generators/inverters proportional to their power ratings or operating costs. The technique relies on using small changes in voltage and frequency to dictate changes in real and reactive power levels. The “phase shift virtual impedance droop control” method is a time-domain implementation of droop control by which the converter is controlled to appear as a virtual AC voltage source in series with a virtual impedance, where the virtual AC voltage source has a constant amplitude and is phase-shifted proportional to the error between the measured grid frequency and the nominal grid frequency.
This technique has several advantages including improved dynamic response and harmonic compensation; however, the method loses direct control of real and reactive currents and thus makes it difficult to impose current limits. Current limits are necessary to constrain the converter to a safe or desired operating region. For example, if the virtual source voltage phasor commanded during droop control would result in a real or reactive current phasor that exceeds the maximum capability of the converter, the converter could be damaged or be forced to shut-down. In addition, by not having direct phasor control of the real and reactive currents, a converter cannot be operated with a virtual impedance while grid connected, resulting in disjointed transitions between islanded and grid-connected states.
Therefore, there is a need in the art for time-domain droop control that includes indirect control of phasor currents.